<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 300px;
            height: 200px;
            background-color: pink;
        }
        .div{
            /* width: 30px;
            height: 30px; */
            /* 用calc()计算出来的宽度永远比父元素小80px */
            width: calc(100% - 80px);
            height:calc(100% - 20px);
            background-color: rgb(62, 224, 110);
        }
    </style>
</head>
<body>
    <div>
    <div class="div"></div>
    </div>
</body>
</html>